Method of manufacturing a fiber-type optical coupler with slanting bragg diffraction gratings

ABSTRACT

A fiber-type optical coupler has two optical fibers including a core on which a slanting Bragg diffraction grating is formed and a first cladding and a second cladding bordered with a boundary plane close to the core. The two optical fibers are placed by approximating the boundary plane almost contacting the core, making respective optical axes almost parallel and also making slanting directions of the respective Bragg diffraction gratings almost parallel. A wave vector of the slanting Bragg diffraction grating is located in a plane made by a normal set up on the boundary plane almost contacting the core and the optical axis of the core, and an angle_made by the wave vector and the optical axis is 0 degree &lt;_&lt;90 degrees. In addition, a refractive index of the second cladding is lower than that of the first cladding.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 09/861,658filed on May 22, 2001, now U.S. Pat. No. 6,870,991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical coupler, optical parts andan optical apparatuses utilizing the optical coupler and also a methodof manufacturing the optical coupler.

2. Description of the Prior Art

An explosive increase in communication traffic is also expected in thefuture due to the advent of an Internet society. And implementation of anetwork capable of accommodating the increase in communication traffic,which is large-capacity, high-speed and inexpensive as to communicationcosts is required. A network technology meeting such requirements is anoptical fiber communication technology. In order to allow largecapacity, high speed and inexpensiveness in the optical fibercommunication technology, it is necessary to develop a multiplexingtechnology of high density. While a time division multiplex technology,a wavelength division multiplex technology and combined use of them arethinkable as a method of multiplexing, a direction to adopt thewavelength division multiplex technology (WDM) is mainstream from aviewpoint of easy extensibility. As a concrete approach, in the backbonesystem (basic network), a WDM technology of such high density aswavelength spacing of zero-point several nm and several tens of GHz forfrequency spacing is developed. In the access system (subscribernetwork) and CATV network, review of methods are underway, such as amethod of utilizing both 1.3-μm wavelength light and 1.55-μm light fortwo-way communication of descending and ascending links and a method ofusing the 1.55-μm light only for a descending link of a broad bandsignal while using the 1.3-μm light for two-way communication. Also, inthe basic network, in addition to a point-to-point communication system,there are an optical add drop multiplexing (Optical ADM) system forputting signals in and out by wavelengths on a node on the way, anoptical cross-connect (Optical XC) system for recombining lightwavepaths and besides, an optical routing system for using wavelengthinformation as address information to determine a destination of opticalsignals and so on so that implementation of a flexible network isexpected.

Thus, the optical parts hold the key to implementation of an advancedoptical communication system that utilizes wavelengths as a resource.One of the especially important optical parts is an opticalmultiplexer-demultiplexer, which multiplexes or demultiplexes lightwaves of different wavelengths to or from a transmission line opticalfiber. Representative multiplexer-demultiplexers for high densitywavelength multiplexing implemented by the conventional technology arean arrayed waveguide grating (AWG) and a fiber Bragg grating (FBG).

Moreover, as for the optical access system, PDS that performs two-wayoptical communication between a station and N (a plurality) subscribersvia 1:N optical star couplers is a representative example of a networksystem. And one of technological challenges of the optical parts is thatthe star coupler sufficiently functions in a descending distributionsystem but ascending signals from the subscriber lines can only collectpower of 1/N at the station in the ascending multiplexing system, whichoccupies a major portion of signal transmission loss, and so an opticalmultiplexer of N:1 capable of optical multiplexing with no loss isanticipated.

In addition, another technology requested to be developed in the opticalaccess system is one that allows, in a high-performance and inexpensivemanner, implementation of an optical transmission and reception modulefor two-way communication to be placed on an optical network unit (ONU)on the subscriber side.

In order to implement the above optical parts such as an opticalcirculator, a 1:N optical coupler and an optical transmission andreception module, it is necessary to develop a new technology that has anonreciprocal transmission property and yet is implemented at low cost.

Furthermore, the optical parts that are important in implementing theoptical communication system are those utilizing the nonreciprocaltransmission property of light. The aforementioned optical circulator isalso one of the representative nonreciprocal optical parts. The opticalcirculator is required not only in the above-mentioned form ofutilization but also in the case of configuring an optical ADM systemfor branching light from transmission lines to nodes (terminalequipment) without loss and inversely inserting light signals from thenodes to the transmission lines.

The above-mentioned conventional multiplexer-demultiplexer devices are adevice system that artfully utilizes optical interference on a waveguideoptical circuit and is configured in a relatively small size with highwavelength resolving power. However, the devices have common faults,that is, they are sensitive to temperature change, increase in opticalinsertion loss due to connection between the devices and optical fiberscannot be ignored, and they are expensive.

In addition, to embody the optical parts and apparatuses that requirethe nonreciprocal transmission property including the opticalcirculator, optical multiplexer with no loss, and optical transmissionand reception module mentioned in the above prior art, it cannot behelped, considering the current technological level, to rely on a methodof using Faraday polarization rotation effect of magneto-opticmaterials. Thus, they must be configured by many discrete elements suchas lenses and magneto-optic crystals so that they are too expensive andunstable to be practical.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a fiber-typeoptical coupler that is an optical device for implementing an advancedoptical communication system utilizing wavelengths as a resource,optical parts of which configuration using this coupler is highlyreliable, stable and economical and a method of manufacturing them aswell as various optical apparatuses utilizing this fiber-type opticalcoupler.

The fiber-type optical coupler of the present invention is comprised ofthe same two optical fibers and couples light from one optical fiber tothe other optical fiber, where the optical fiber has a core on which aslanting Bragg diffraction grating is formed and two claddings ofdifferent refractive indexes bordered with a plane parallel with anoptical axis of the core and almost contacting the core. A wave vectorof the slanting Bragg diffraction grating is located in a plane made bya normal set up on the border of the plane almost contacting the coreand the optical axis, where an angle θ made by the wave vector and theoptical axis is 0 degree <θ<90 degrees, and the two optical fibers areplaced by approximating the plane almost contacting the core, making therespective optical axes almost parallel and also making slantingdirections of the respective Bragg diffraction gratings almost parallelso that, as for the refractive indexes of the two claddings borderedwith the plane almost contacting the core, the refractive index of thecladding included in the area where the core exists from the plane ishigher than that of the cladding included in the area where the coredoes not exist.

A manufacturing method of the fiber-type optical coupler of the presentinvention has a first process of forming, in the optical fiber of whichcore is surrounded by claddings, a Bragg diffraction grating by periodicchange of the refractive index whereby angle θ made by the wave vectorand the optical axis of the optical fiber is 0 degree <θ<90 degrees, asecond process of forming, in a section vertical to the plane made bythe wave vector and the optical axis and also vertical to the opticalaxis of the optical fiber, a first Bragg diffraction grating fiberhaving a first cladding and a second cladding of which refractive indexis lower than that of the first cladding, bordered with a line drawn byapproximating the core, and a third process of placing the first Braggdiffraction grating fiber and a second Bragg diffraction grating fiberhaving the same configuration as the first Bragg diffraction gratingfiber by making the respective optical axes almost parallel and alsomaking slanting directions of the respective Bragg diffraction gratingsalmost parallel and also approximating boundary planes of the first andsecond claddings. And in the third process, the first and second Braggdiffraction grating fibers are accommodated and fixed in grooves formedon substrates respectively, and the respective substrates have means forplacing the first and second tilt Bragg grating fibers by making therespective optical axes thereof almost parallel and approximatingboundary planes of the respective first and second claddings.

In addition, the optical part using the fiber-type optical coupler ofthe present invention has a plurality of fiber-type optical couplers ofdifferent wavelengths to meet Bragg conditions of the Bragg diffractiongratings and are concatenated so that the slanting directions of theBragg diffraction gratings become the same as the direction of opticaltransmission in the plurality of fiber-type optical couplers. Thisoptical part performs operation of a multiplexer-demultiplexer. Inaddition, the optical part having a plurality of fiber-type opticalcouplers meeting Bragg conditions of the Bragg diffraction gratingsshows a property of optical multiplexing with no loss.

The other optical part using the fiber-type optical coupler has N tiersof (N is a positive integer of 2 or more) fiber-type optical couplers ofthe same wavelengths to meet Bragg conditions of the Bragg diffractiongratings. And in an optical input-output state wherein the light isinputted from one fiber terminal of the first optical fiber of thefiber-type optical coupler and the light is outputted from the secondoptical fiber thereof, if the optical input terminal of the firstoptical fiber is a terminal A, the other terminal of the first opticalfiber is a terminal C, and the optical output terminal of the secondoptical fiber is a terminal B, then the terminal A of the fiber-typeoptical coupler on each tier is an optical input-output port, theterminal B of the fiber-type optical coupler on the N=i-th tier isconnected to the terminal C of the fiber-type optical coupler on theN=i+1-th tier, and the terminal B of the fiber-type optical coupler onthe last N=N-th tier is connected to the terminal C of the fiber-typeoptical coupler on the N=1-th tier. This optical part performs operationof an optical circulator. Moreover, the optical part of which opticalinput port is the terminal C of the fiber-type optical coupler andoptical output port is the terminal A performs operation of an opticalisolator.

Furthermore, as for the optical apparatus using the fiber-type opticalcoupler of the present invention, a semiconductor laser for transmissionis connected to the terminal C of the fiber-type optical coupler, theterminal A thereof is the optical input-output port to an opticaltransmission line, and a photo-detector for reception is connected tothe terminal B thereof. This apparatus performs operation of an opticaltransmitter/receiver.

The other optical apparatus has an optical amplification fiber fordirectly amplifying signal light, a pumping source for optically pumpingthe optical amplification fiber, the first and second fiber-type opticalcouplers for making the wavelength of the signal light meet the Braggcondition of the Bragg diffraction grating and the third fiber-typeoptical coupler for making the wavelength of the pumping source meet theBragg condition of the Bragg diffraction grating, where the terminal Aof the first fiber-type optical coupler is the input terminal of thesignal light, the terminal B is connected to the terminal C of the thirdfiber-type optical coupler, the terminal A of the third fiber-typeoptical coupler is connected to the pumping source, the terminal B ofthe third fiber-type optical coupler is connected to one terminal of theoptical amplification fiber, and the other terminal of the opticalamplification fiber is connected to the terminal A of the secondfiber-type optical coupler, and the terminal B of the second fiber-typeoptical coupler is a signal output terminal. This apparatus performsoperation of an optical amplifier. Moreover, it has the second pumpingsource and the fourth fiber-type optical coupler for making the outputwavelength of the second pumping source meet the Bragg condition of theBragg diffraction grating, where the other terminal of the opticalamplification fiber is connected to the terminal B of the fourthfiber-type optical coupler, the second pumping source is connected tothe terminal A of the fourth fiber-type optical coupler, and theterminal C of the fourth fiber-type optical coupler can also beconnected to the terminal A of the second fiber-type optical coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionwhen taken with the accompanying drawings in which:

FIG. 1 is a diagram showing a configuration of an AWG device that is oneof conventional multiplexer-demultiplexers;

FIG. 2 is a diagram showing a configuration of the conventionalmultiplexer-demultiplexer comprised of a fiber grating and an opticalcirculator;

FIG. 3 is a diagram showing a structure of the multiplexer-demultiplexerthat is a first embodiment of the present invention;

FIGS. 4A and 4B are diagrams showing structures of a fiber-type opticalcoupler that is a basic device for implementing various optical partsand apparatuses for optical communication of the present invention;

FIG. 5 is a drawing showing a wave number diagram for describingoperation of the fiber-type optical coupler of the present invention;

FIGS. 6A and 6B are diagrams for describing how to form a grating of thefiber-type optical coupler of the present invention;

FIGS. 7A, 7B and 7C are diagrams for describing how to assemble andimplement the fiber-type optical coupler of the present invention;

FIGS. 8A and 8B are diagrams showing another configuration of a siliconV groove used for assembling and implementing the fiber-type opticalcoupler of the present invention;

FIGS. 9A and 9B are diagrams showing a structure of a module of themultiplexer-demultiplexer using the fiber-type optical coupler of thepresent invention after assembly and implementation;

FIG. 10 is a block diagram of an optical fiber amplifier using thefiber-type optical coupler, which is a second embodiment of the presentinvention;

FIGS. 11A, 11B, 11A′ and 11B′ are diagrams for describing operatingprinciples of an optical irreversible transmission property of thefiber-type optical coupler of the present invention;

FIGS. 12A, 12B, 12A′ and 12B′ are diagrams for describing operatingprinciples of the optical irreversible transmission property of thefiber-type optical coupler of the present invention;

FIGS. 13A and 13B are diagrams for describing operating principles ofthe optical irreversible transmission property of the fiber-type opticalcoupler of the present invention;

FIGS. 14A and 14B are diagrams of a configuration and a structure of anoptical transmitter/receiver module using the optical reversibletransmission property of the fiber-type optical coupler, which is athird embodiment of the present invention;

FIG. 15 is a block diagram of the transmitter/receiver by means of 1wave using the same wavelength for transmission and reception by use ofthe fiber-type optical coupler, which is a fourth embodiment of thepresent invention;

FIG. 16 is a block diagram of an optical multiplexer with no loss usingthe optical irreversible transmission property of the fiber-type opticalcoupler, which is a fifth embodiment of the present invention;

FIG. 17 is a block diagram of a 3-port optical circulator using theoptical irreversible transmission property of the fiber-type opticalcoupler, which is a sixth embodiment of the present invention; and

FIG. 18 is a block diagram of the optical fiber amplifier using theoptical irreversible transmission property of the fiber-type opticalcoupler, which is a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an arrayed waveguide grating (AWG) that is aconventional optical multiplexer-demultiplexer is configured byintegrating input waveguides, input slab waveguides, arrayed waveguides,output slab waveguides and output waveguides through the use of a silicaglass plane waveguide production technology on a silicon substrate. Itsoperating principles are similar to those of a spectroscope. The arrayedwaveguide is comprised of a large number of channel waveguides, wheredifferences in length of adjacent channel waveguides are set at severaltens of μm or so. This group of channel waveguides having optical lengthdifferences play a role of a diffraction grating in the ordinaryspectroscope.

Signal light including a large number of wavelengths led to the inputwaveguide is radiated to the input slab waveguides and then distributedto arrayed waveguides. The distributed signal light is divided into alarge number of the channel waveguides and radiated to the output slabwaveguides and comes into a focus at a terminal of the outputwaveguides. When transmitted on the channel waveguides, however, it isdelayed differently if wavelengths are different, and so it comes into afocus at a terminal of a different output waveguide depending on thewavelength, and is radiated out of the output waveguide. Thus,demultiplexer operation is performed. Inversely, due to reciprocity, iflight of a large number of wavelengths is let the into input waveguidesarranged in order of wavelengths, multiplexer operation is implementedbecause it is multiplexed on specific output waveguides and isoutputted.

In addition, FIG. 2 is an example of a WDM device using a fiber gratingthat is a conventional and different optical multiplexer-demultiplexer.It is a demultiplexer configured by combining optical circulators (OC)of three terminals marked A, B and C with the fiber grating (FG) thatreflects light of a specific wavelength. It can connect to a terminal Ban FG for inputting the multiplexed light from a terminal A of theoptical circulator and having light of a desired wavelength reflectedand transmitting any other wavelength to a next tier so as to tap andoutput light of a specific wavelength reflected by the FG from aterminal C. It is also possible to configure a multiplexer by changingthe connection method.

There are the following problems to the conventional opticalmultiplexer-demultiplexer. In the device in FIG. 1 using an AWG, atransmission wavelength property of the device deteriorates unlessmutual phase relationship of the light transmitted on the large numberof channel waveguides comprising the arrayed waveguides are alwaysstrictly kept. Nevertheless, at the time of producing the waveguides,there arises influence of fluctuations of a section size and arefractive index and also double refraction based on a distortiongenerated between it and a substrate. Thus, the phase relationship ofthe light transmitted on the channel waveguides collapses so that thetransmission wavelength property of the device deteriorates andcrosstalk is generated between the channels. For this reason, it isdifficult to produce the device with high yields.

In addition, in the device in FIG. 2 using a fiber grating, a principleof having guided mode light of a specific wavelength reflected from aforward traveling wave to a backward traveling wave is used. Thisprinciple is also used for certain optical fiber sensors for detectingtemperature and pressure in a highly sensitive manner, where a fault isthat its reflection property is extremely sensitive to environmentalvariation such as temperature and pressure and besides, it is also afault that the optical circulators to be used by a large quantity inorder to configure the device are high-loss and high-cost.

Referring to FIG. 3, the multiplexer-demultiplexer of a first embodimentof the present invention comprises a principal optical fiber 1 and npieces of branching fiber 2-1 to 2-n, and Bragg gratings 3-1 to 3-nformed tilting to an optical axis are provided in cores at n locationspartway in the direction of optical transmission of the principaloptical fiber 1, and also Bragg gratings 4-1 to 4-n formed tilting tothe optical axis are provided in the cores near one terminal of thebranching fibers 2-1 to 2-n. Periods of the n pieces of tilt gratingsΛ₁, Λ₂, Λ₃, . . . Λ_(n) are all different.

And as shown in FIG. 4, the principal optical fiber 1 and the branchingfibers 2-1 to 2-n are placed at areas forming the respective Bragggratings in the optical axis direction with mutual optical axes paralleland the respective cores close. FIG. 4A shows a section view cutvertically to the optical axis and FIG. 4B shows a section view cutalong the optical axis. As shown in FIG. 4A, a cladding 1 b of theprincipal optical fiber is removed up to a boundary with the principaloptical fiber core 1 a, and a cladding 2-1 b of the branching opticalfiber is also removed up to a boundary with the branching optical fibercore 2-1 a so that they are placed facing a removed side to each otherwith mutual optical axes parallel and the cores close.

And as shown in FIG. 4B, in physical relationship of the optical axisdirections in which two fibers are closely placed, the areas formingtilt gratings respectively are located along the optical axis in anoverlapping manner, and inclination θ of each individual tilt grating isin the plane made by a normal set up on the plane created by removingits portion including its cores and the optical axis of the opticalfibers. A tilt grating 3-1 of the principal optical fiber 1 and a tiltgrating 4-1 of the branching fiber 2-1 which are facing each other havethe same period of grating and also the same tilt angle θ.

Operation of the first embodiment of the present invention will bedescribed by referring to the section view of FIG. 4B that is cut alongthe optical axis and FIG. 3. In FIG. 4B, signal lights λ₁, λ₂, λ₃, . . .λ_(n), that are wavelength-multiplexed n waves entering into theprincipal optical fiber core 1 a from the left terminal of thisprincipal optical fiber advance into the tilt grating 3-1, and then onlythe light of a wavelength λ₁ of the n waves is Bragg-diffracted by thetilt grating 3-1 of a period Λ₁, wave number K₁ (=2n/Λ ₁) created in theprincipal optical fiber core. The Bragg condition is a condition whereinthree vectors, that is, a wave number vector k_(f1) of the incidentlight, a wave number vector K₁ of the tilt grating and a wave numbervector k_(a1) of the diffraction light diffracted to the air form aclosed triangle. To be more specific, diffraction is made to a directionof an angle φi satisfying both of the following expressions.K ₁ cos (θ₁)=k _(f1) +k _(a1) cos (φ₁)  (1)K ₁ sin (θ₁)=k _(a1) sin (φ₁)  (2)FIG. 5 shows a relationship satisfying the above expressions (1) and(2). If the tilt grating is of sufficient length such as several tens ofmillimeters, then Bragg diffraction occurs, and light of ahundred-percent wavelength λ_(i) mode on principles is diffracted in theair and any mode other than that wavelength propagates in the principaloptical fiber core. In this connection, the tilt angle θ is 17 to 18degrees or so at a wavelength of 1.55 μm.

Returning to FIG. 4B, the light of wavelength λ₁ diffracted by the tiltgrating 3-1 enters into the tilt grating 4-1 existing sandwiching aslight gap, which is a branching optical fiber of the same structure asthe principal optical fiber tilt grating 3-1. As the tilt gratings 3-1and 4-1 are of the same structure, the Bragg conditions of theexpressions (1) and (2) are also satisfied in the tilt grating 4-1 sothat the incident light to the tilt grating 4-1 is diffracted by ahundred percent to be converted into the guided mode of the branchingfiber 2-1. This configuration is similar to configuration wherein bothtilt gratings are microwave transmission and reception antennas such asa horn reflector antenna and parabola antenna, and each individual corewaveguide leading light to and having light led from the tilt gratingsplays a role of each individual feeding waveguide and receivingwaveguide.

The distance between the principal optical fiber core 1 a and thebranching optical fiber core 2-1 a shown in the section view of FIG. 4Ais long enough to the extent that a seepage of the guided modetransmitted in the core 1 a into the air does not go as far as the core2-1 a, that is, distant enough for both waveguides not to be opticallycoupled and yet for the light diffracted by the tilt grating 3-1 toenter almost entirely into the core 2-1 a of the branching optical fiber2-1 without widening, which value is approximately several μm.

In the embodiment of FIG. 3 wherein a plurality of couplers made up ofpairs of such tilt gratings are formed, only the light of wavelength λ₁out of the light that was wavelength-multiplexed and entered into theprincipal optical fiber 1 is branched into the branching optical fiber2-1. Likewise, only the light of the wavelength λ₂ can be branched intothe branching optical fiber 2-2, and furthermore, the light of thewavelength λ_(n) can be branched into the branching optical fiber 2-n.To be more specific, a demultiplexer for isolating light of differentwavelengths is configured.

In FIGS. 3 and 4B, the light of wavelength λ₁ incident from the leftterminal of the principal optical fiber and advancing rightward isdiffracted by a pair of tilt gratings 3-i, 4-i (i=1 to n) ofconfiguration meeting the expressions (1) and (2), and the route foradvancing rightward on the branching optical fiber 2-1 is reversible, sothat in the inverse direction, that is, the light of wavelength λ₁incident from the right terminal of the branching optical fiber 2-i isdiffracted by a pair of tilt gratings 4-i, 3-i to be coupled with guidedlight of the principal optical fiber 1 advancing leftward. To be morespecific, the embodiment in FIG. 3 also has a function of themultiplexer.

The multiplexer-demultiplexer of the present invention has almost nodependency on polarization. To be more specific, as shown in FIG. 4A,because the core 1 a or 2-1 a exposes a surface partially in the air andis mostly surrounded by claddings, its non-axis symmetry property ofboundary conditions against a waveguide layer is extremely small. Thus,the dependency on polarization by the Bragg grating is little, and so adiffraction property of almost no dependency on polarization can beacquired. As this configuration has its grating formed in a slantingdirection to the optical axis so that reflection and transmissionproperty of a plane wave has dependency on polarization to dielectricmultiplayer, in the case where a difference in the Bragg condition(phase matching condition) arises between the core guided mode thatpolarizes in parallel to the plane made by the optical axis and agrating wave number vector K and the core guided mode that polarizesvertically resulting in a difference in optical transfer properties, itis possible to secure no dependency on polarization by providing twotilt gratings having slightly different tilt angles and periods alongthe optical axis of the core.

Next, a method of production and assembly of the above described firstembodiment will be described by referring to the drawings. FIGS. 6A and6B are diagrams showing how to produce a fiber tilt grating, where FIG.6A is a front view, and FIG. 6B is a perspective drawing viewing only agrating creation mask and an optical fiber in a plane manner.Ultraviolet light 17 oscillated by a KrF excimer laser 10 is applied toa fiber grating creation phase mask 12. The phase mask uses a nitratedmaterial of high permeability to ultraviolet light of fused quartz andso on, and has an uneven grating of which section is a rectangle. Alevel difference between concave and convex portions is formed so that aphase difference becomes an integral multiple of a ½ wavelength to thewavelength of the ultraviolet light 17 and width becomes 1:1 between theconcave and convex portions. The light incident in the phase mask 12 ofwhich phase difference is a ½ wavelength and ratio between the concaveand convex portions is 1:1 is diffracted by the phase mask so as to beemitted only as + primary diffraction light 13 and − primary diffractionlight 14. They interfere to form interference fringes 15. Where theinterference fringes 15 were formed, to make the refractive index of thecore section higher than that of the cladding section, an ordinarysilica communication single mode optical fiber 11 that is doping Ge isplaced in the core section. As for placement relationship of the phasemask 12 against the optical fiber 11, the phase mask 12 is rotated andplaced so that the interference fringes 15 are inclined against theoptical axis of the optical fiber 11 just by the angle θ on planeplacement as shown in FIG. 6B. As with conventional FBG creation,ultraviolet light is irradiated to the optical fiber 11 so as to form agrating arising from generation of a color center of Ge in the coresection. And then, the phase mask is replaced with one having adifferent grating pitch, and one optical fiber is moved to anotherlocation by sliding or a fiber rod cut short in advance is replaced soas to form tilt gratings of different Bragg diffraction wavelengths oneafter another.

Next, a method of forming a plane contacting the core surface on theoptical fiber that has thus formed a tilt-shaped fiber grating will bedescribed. As aforementioned, this surface is parallel with the opticalaxis of the optical fiber and also vertical to the plane including theoptical axis and the tilt angle of the tilt grating.

FIG. 7 shows a structure of an optical fiber supporting member using asilicon wafer forming V grooves and fiber arrangement to it. This playsa role of both a “spline” and an “assembly jig” for forming a planecontacting the core surface. FIG. 7A is a plan view, B is a front view,and C shows a section view at a cutting line A–B.

On the substrate of Si wafer 20, a plurality of V grooves 21 and 22 areformed in parallel each other. The V grooves 21 is a groove supportingthe optical fiber forming the tilt grating, and the V grooves 22 is agroove supporting a dummy fiber 40. The dummy fiber 40 forms a planereaching the core surface on the optical fiber forming the tilt grating,and in a later process of assembling two sets of optical fiber linesformed in section shape that may be called D-shape by joining the aboveplanes as shown in FIG. 4A, it plays a role of a guide, so to speak, tosecure precision of parallelism and proximity between the upper andlower cores.

Optical fibers 30 forming the tilt grating are placed in line in the Vgroove 21. The optical fibers 30-1, 30-2 and 30-3 are forming the tiltgratings of different Bragg wavelengths, and the placement relationshipbetween the tilt direction of the gratings and the Si wafer principalplane is that, as shown in FIG. 7C, the plane including the optical axisand the tilt angle direction is placed to be orthogonal to the Si waferprincipal plane. The optical fibers 30 can be placed on the V groove 21by folding one fiber forming a plurality of the tilt gratings in theaxial as one piece, or those cut short in advance can be separatelyplaced.

Such a jig is used to polish the optical fibers 30, that is, to polishthe plane by using a mechanochemical method (MC polishing) ofconcurrently using chemical etching and mechanical polishing until thecore surface is just exposed. At this time, it is also possible to formfilm 50 that plays a role of a polishing stopper to the MC polishing andset in advance a width dimension W1 of the V groove on which the opticalfibers 30 are to be placed so that, when the polishing plate reaches thepolishing stopper and the polishing is stopped, the polishing hasspontaneously reached the core surfaces of the optical fibers 30.

It is also possible to make the shape of the V groove for accommodatingthe optical fibers as shown in FIG. 8. To be more specific, just asshown in the plan view in FIG. 6A, it is possible to form the width ofthe V groove supporting the optical fiber narrowly at the center of thesubstrate Si and form it widely ahead of and behind the center so as tomount the optical fiber as arched on the whole by making it high at thecenter and deep down in the substrate Si ahead of and behind the centeras shown in FIG. 8B, thereby allowing to polish only the area requiringpolishing.

In addition, it is further desirable after this plane polishing toperform antireflection coating by a single-layer or multilayerdielectric film in order to eliminate Fresnel reflection on the polishedsurface, between the quartz that is a matrix of the optical fiber andthe air.

Next, an assembling method will be described, where a principal opticalfiber and a branching optical fiber having tilt gratings and exposingthe core are approximated to be assembled into the opticalmultiplexer-demultiplexer in FIG. 3. FIG. 9 shows an example of amultiplexer-demultiplexer comprised of two fibber arrays 60-1 and 60-2forming three tilt gratings of different Bragg wavelengths and havingcompleted the above polishing. FIG. 7A shows a completedmultiplexer-demultiplexer for three waves, and FIG. 7B shows a sectionview at the cutting line A–A.′

A D-shaped fiber array made by 31-1, 31-2 and 31-3 forming tilt gratingsand a D-shaped fiber array made by 32-1, 32-2 and 32-3 are facing eachother. While the Bragg wavelengths are different among the respectivesets, the fibers facing each other have tilt gratings of the same Braggwavelengths formed. In order to secure the parallelism of the opticalaxes of both cores, horizontal relative physical relationship betweenthe formed tilt gratings and an adequate gap between fiber arrays 60-1and 60-2, a dummy fiber 40 for guide is inserted into the V grooveprovided at both ends of the arrays so as to fix the two fiber arrays60-1 and 60-2 as one piece by organic adhesion or metallic fusion.

After that, in the case where the tilt grating fibers placed on the Vgroove for the purpose of polishing for forming the aforementioned planeare linked as one, the optical fiber of the fiber array 60-2 on thebranching side in FIG. 7A is cut to expose its end face. In addition, ifthe tilt grating fibers placed on the V groove are cut one by one, asplice for linking principal optical fibers of the fiber array 60-1 intoone piece is conducted. The above process completes the opticalmultiplexer-demultiplexer of the first embodiment of the presentinvention.

Moreover, as the gap made between the upper and lower fibers just has tobe of a lower refractive index than the fiber cladding, it is alsofeasible to fill it with the air as it is or a transparent low molecularor high molecular resin of lower refractive index than quartz such as afluorine-inclusive resin.

Furthermore, in the case where variances arise as to productionprecision of the V groove provided for inserting the dummy fiber 40 forguide or a diameter of the dummy fiber 40 itself so that the distancebetween the planes of fiber arrays 60-1 and 60-2 may not be setcorrectly, it is also possible, when assembling a liquid crystal panel,to disperse spacers to be inserted between two sheets of glass to becemented together in the areas other than the cores.

As mentioned above, the optical coupler configuration between theoptical fibers by the tilt gratings of the present invention uses silicacommunication single mode optical fibers as its matrix, which isproduced by strictly controlling optical propagation properties, and soit can be manufactured with stable yields without varying Bragg matchingcondition depending on a lot of the optical fibers to be used.

Next, a fiber optical amplifier of a second embodiment of the presentinvention will be described by referring to the drawings. FIG. 10 is anembodiment where in the present invention is implemented to a fiberoptical direct amplifier, and it comprises an optical amplificationrare-earth dope fiber 70 containing rare-earth elements, an excitationlaser diode 74, an excitation light coupling tilt Bragg grating 71-1provided to an input fiber on a light signal input side of therare-earth dope fiber 70, a tilt Bragg grating 71-2 provided to anexcitation light output fiber forming a pair therewith, a signal lightoutput tilt Bragg grating 72-1 provided on an output side of therare-earth dope fiber 70 and a tilt Bragg grating 72-2 provided to asignal extraction fiber forming a pair therewith.

Configuration of a pair of the fiber tilt Bragg gratings is the same asthe configuration in FIG. 4, and it is produced and assembled by themethod described in detail so far.

The tilt Bragg grating 71-1 and 71-2 that form a pair couple excitationlight 74 having a different wavelength from signal light 73 to therare-earth dope fiber 70. As the signal light 73 does not satisfy theBragg wavelength, it enters into the rare-earth dope fiber 70 bytransmitting through the grating without getting diffracted by it. Forinstance, in the case where erbium is used as a rare-earth element,light of around 1.55-μn wavelength is often used as signal light, andlight of 1.49 μm or 0.98 μm is often used as excitation light. The tiltgratings 72-1 and 72-2 that form a pair on the output side of therare-earth dope fiber 70 output light of signal light wavelength 75after performing Bragg diffraction and eliminates amplified spontaneousemission (ASE) 76 that becomes noise light from the optical amplifier.Thus, the multiplexing and demultiplexing properties of the opticalcoupler created by the pair of tilt gratings of the configuration of thepresent invention are effectively utilized. To be more specific, aremarkably matching optical circuit comprised only of optical fiberconfiguration requiring no optical coupling parts such as a lens formultiplexing and demultiplexing can be configured.

While the embodiment of FIG. 10 describes a case of a forward excitationwherein optical excitation occurs ahead of the rare-earth dope fiber, itis also effective in the case of backward excitation and excitationoccurring both forward and backward.

Next, a further embodiment of the present invention will be described.Before that, further unique properties of basic components of thepresent invention will be described. This description will helpunderstand the third embodiment and thereafter that utilize the uniqueproperties.

The basic components of the present invention are, so to speak, a set ofBragg diffraction optical couplers of configuration wherein two opticalfibers forming a tilted Bragg grating in the core section are placedwith their optical axes parallel and the respective cores close in thearea forming mutual tilt gratings. In the placement related to apropagation direction of guided mode light and a tilt angle of the Bragggrating described in detail so far, this device causes Bragg diffractioncoupling from the guided mode for letting it in from the left terminalof one primary optical fiber and advancing it rightward in the coresection to the guided mode for also advancing it rightward in the othersecond optical fiber core via the tilt Bragg grating, and then emits itfrom the right terminal of the secondary primary optical fiber. Asreciprocity holds as to the propagation during this process, and so iflight is let in from the other secondary right terminal, it is emittedfrom the left terminal of the primary optical fiber.

In case of thinking of the tilt angle of the above Bragg grating asfixed, however, the guided mode light incident from the right terminalof the primary optical fiber and advancing leftward is not diffracted bythe grating since there is no radiation mode allowing a Bragg matchingcondition to hold and only advances leftward to transmit and propagatein the core of its own primary optical fiber. In addition, the modelight incident from the left terminal of the other secondary opticalfiber and advancing rightward is not diffracted by a grating alsobecause no Bragg matching condition holds and only advances rightward totransmit and propagate in the core of its own secondary optical fiber.To be more specific, it is characterized by showing a differenttransmission property depending on the terminal from which it enters theoptical fiber core.

Unique operation of the Bragg diffraction coupling elements between theabove tilt Bragg gratings will be described by referring to FIGS. 11 and12. As shown in FIG. 11A, when the guided mode k_(f) entering into theprincipal optical fiber side from the left terminal and advancingrightward advances into the tilt grating, the guided mode k_(f)undergoes Bragg diffraction by the tilt grating and couples with theradiation mode k_(a) on the air side. Inversely, if light of wave numberk_(a) enters into the tilt grating at a Bragg angle from the air side,it is converted into the guided mode k_(f) advancing leftward in theprincipal optical fiber. This is the reciprocal operation described inthe first embodiment shown in FIG. 3. FIG. 11B is a diagram describingphase matching conditions in a wave number space.

As opposed to this, FIG. 11A′ shows that, when the guided mode k_(f)entering into the principal optical fiber shown by the wave line fromthe right terminal and advancing leftward advances into the tiltgrating, radiation light k_(a) to the air side is not generated and itpropagates and transmits as the guided mode k_(f) without beingdiffracted by the grating. It can be understood from the phase matchingcondition in the wave number space of FIG. 11B. If the wave vector ofthe guided mode indicated by the wave line currently transmittingleftward in the wave number space enters into the grating, it cannotpump radiation light of the size of the wave number in the air in thesolid line via a wave vector K of the tilt grating in the double line.It is because a cladding section of a higher refractive index than theair exists above the grating but there is no air, since light k_(g)(>k_(a)) of the wave number propagating in the cladding section cannotbe pumped by the grating.

Likewise, in FIG. 12, FIG. 12A shows the reciprocal operation describedin the description of operation of the first embodiment wherein, whenthe light of Bragg wavelength wave number k_(a) enters into the tiltBragg grating on the branching optical fiber side of FIG. 3 at the Braggangle from the air side, it undergoes Bragg diffraction by the tiltgrating, couples with the guided mode k_(f) of the branching opticalfiber, and if the guided mode k_(f) inversely enters into the tiltgrating from the right side, it is converted into radiation light to theair k_(a), and FIG. 10B is a diagram describing the phase matching inthe wave number space.

As opposed to this, FIG. 12A′ shows that, when the guided mode k_(f)enters from the left terminal of the branching optical fiber shown bythe wave line, radiation light to the air side is not generated and ittransmits and propagates as-is as the guided mode k_(f) without beingdiffracted by the grating. This phenomenon can be understood from thephase matching condition in the wave number space of FIG. 12B′. If thewave vector of the guided mode k_(f) indicated by the wave linecurrently transmitting rightward in the wave number space enters intothe grating, it cannot pump radiation light of the size of the wavenumber k_(a) in the air in the solid line via the wave vector of thetilt grating in the double line. It is because a cladding section of ahigher refractive index than the air exists below the grating but thereis no air. That is because the light of the wave number k_(a)propagating in the cladding section does not meet the phase matchingcondition and so it cannot be pumped by the grating K.

Thus, if a set of Bragg diffraction-shaped light couplers ofconfiguration wherein two optical fibers having tilted Bragg gratingsformed in their core sections are placed with their optical axesparallel and the respective cores close in the areas forming mutual tiltgratings, as shown in FIG. 13A, the guided mode entering into the uppercore shown by the solid line from a left terminal A is diffracted by theBragg grating, couples with the guided mode of the lower core andadvances rightward to be outputted from a right terminal D. And as botharrows in the solid line indicate, this optical path is reversible. Asopposed to this, as shown by the dashed lines, the light incident from aright terminal C on the opposite side to the solid line of the uppercore is not diffracted by the Bragg grating, and transmits andpropagates in its core as the guided mode to be outputted from the leftterminal A. Likewise, the light entering into the lower optical fiberfrom a left terminal B transmits through its core in a straight line andis emitted from the right terminal D. The circuit in FIG. 13B is theoptical circuit of FIG. 13A represented as a four-terminal. Thus, theoptical coupler of the present invention shows an irreversible transferproperty.

It is possible to implement a useful directional device by utilizing theabove irreversible optical transmission property of the optical couplerof the present invention.

FIG. 14 shows the multiplexer-demultiplexer using two waves of a thirdembodiment of the present invention. FIG. 12A shows a configurationprinciple diagram, and FIG. 12B shows an assembly diagram.

In FIG. 12A, a principal optical fiber 100 is coupled with a branchingoptical fiber 102-1 for receiving the wavelength λ₁ by a Braggdiffraction optical coupler 106-1 by the tilt grating, and is alsocoupled with a branching optical fiber 102-2 for receiving thewavelength λ₂ by a Bragg diffraction optical coupler 106-2. In addition,a branching optical fiber 103-1 for sending the wavelength λ₁ is coupledwith the principal optical fiber 100 by a Bragg diffraction opticalcoupler 107-1 having the same properties as that for receiving thewavelengths λ₁. Moreover, a branching optical fiber 103-2 for sendingthe wavelength λ₂ is coupled with the principal optical fiber 100 by aBragg diffraction coupler 107-2 having the same properties as that forreceiving the wavelength λ₂.

If the light with the wavelengths λ₁ and λ₂ multiplexed incident from anoptical input-output terminal at the left terminal of the principaloptical fiber 100 enters into the Bragg diffraction optical coupler106-1, only the wavelength λ₁ is led by the branching optical fiber102-1 and undergoes photoelectric conversion at a light receiving device104-1 according to the principle described in detail so far. The lightin the principal optical fiber 100 that became only the wavelength λ₂ bydemultiplexing the light of the wavelength λ₂, is demultiplexed by theBragg diffraction coupler 106-2 in which the Bragg wavelength formed bythe principal optical fiber 100 and the branching optical fiber 102-2 istuned to λ₂, and is received by the receiving device 104-2.

On the other hand, the Bragg diffraction optical coupler 107-1 providedto the principal optical fiber for multiplexing of which Braggwavelength is tuned to λ₁ and the Bragg diffraction optical coupler107-2 of which Bragg wavelength is tuned to λ₂ couple the respectiveoutput of a semiconductor laser 105-1 oscillating at the wavelength λ₁and a semiconductor laser 105-2 oscillating at the wavelength λ₂ withthe principal optical fiber 100 as a guided mode advancing leftward.Even if the light of the wavelength λ₁ coupled with the principaloptical fiber 100 reaches the aforementioned branching Bragg diffractionoptical coupler 106-1 tuned to this wavelength, it is not coupled withthe branching optical fiber for receiving 102-1 due to theabove-mentioned nonreciprocity and advances in the principal opticalfiber 100. Likewise, even if the light of the wavelength λ₂ coupled withthe principal optical fiber 100 reaches the aforementioned branchingBragg diffraction optical coupler 106-2 tuned to this wavelength, it isnot coupled with branching optical fiber for receiving 102-2 due to theabove-mentioned nonreciprocity and advances in the principal opticalfiber 100 in a state of being multiplexed with the light of thewavelength λ₁.

This multiplexer-demultiplexer can be completed by the methods ofproduction and assembly shown in the first embodiment, as with thedemultiplexer shown in FIG. 9A, by combining two V grooves supportingthe fiber with supporting plates 110-1 and 110-2 and splicing theoptical fiber as required as shown in FIG. 14B.

As the multiplexer-demultiplexer of this embodiment can be produced atlow cost, it is quite effective if used as an optical componentcomprising a terminating unit of the aforementioned access network andCATV network. Conventionally, plane-type silica and plastic wave guidedevice shave been studied as optical multiplexing-demultiplexing partsfor 2-wavelength two-way transmission (optical transceiver) of the lightof 1.3 μm and 1.55 μm, and yet, it has been difficult to performsuccessful communication since transmission light directed to an up linkof high intensity diffracts to the receiving device for a down receivinglink and receiving signals are masked. As for themultiplexing-demultiplexing device of this embodiment, the above problemof crosstalk does not occur since a transmission LD and the receivingdevice are easily separable optically.

While the third embodiment was described based on two waves, it is easyto extend it to an optical multiplexer-demultiplexer for a superhigh-density wavelength multiplexing transmission system forwavelength-multiplexing several tens or several hundreds of waves intoone optical fiber and transmitting. In addition, it can be applied notonly to a communication system of a basic network but also widely to anoptical add drop multiplexing (Optical ADM) system for putting signalsin and out by wavelengths on a node on the way, an optical cross-connect(Optical XC) system for recombining wavelength paths and besides, anoptical routing system of new network configuration for usingwavelengths as address information to determine a destination of opticalsignals.

Inversely, a useful device can also be acquired in the case of one wave.FIG. 15 shows configuration of a transmitter/receiver of one wave usingthe same wavelength for transmission and reception according to thefourth embodiment of the present invention. It is configured by a Braggdiffraction optical coupler 90 connected to a communication line 95, aphotoreceiver 94 for receiving a down signal λdown91 and an LD93 forreceiving an up signal λup92. The down signal λdown91 and the up signalλup92 have almost the same wavelength. As described as the nonreciprocaloptical transfer property of this Bragg diffraction optical coupler inFIG. 13, the down signal λdown91 is optically transferred between thefibers by Bragg diffraction in this Bragg diffraction optical coupler 90and is led to the photoreceiver 94 for receiving, whereas the up signalλup emitted from the LD93 transmits through the Bragg diffractionoptical coupler 90 in a straight line and is coupled with thecommunication line 95. Thus, this Bragg diffraction optical couplerallows two-way communication by using the same optical fibertransmission line, and what is more, by one wave.

Moreover, from another viewpoint, if FIG. 15 is regarded merely as aone-way communication transmitter of λup rather than two-waycommunication and λdown as reflected returning light from the LD93transmission line, the reflected returning light is inhibited fromentering into the LD, and therefore it can also be said that the Braggdiffraction fiber optical coupler 90 itself plays a role of an isolator.

Next, it is further possible, by utilizing the nonreciprocity of theBragg diffraction optical coupler by the tilt grating of the presentinvention, to couple a plurality of light of the same wavelength to oneoptical fiber with no loss so as to implement an optical multiplexerwith no loss.

As a fifth embodiment, FIG. 16 shows a configuration of the opticalmultiplexer with no loss. FIG. 16 indicates the optical multiplexer withno dependency on polarization and no loss wherein output of n pieces ofLD301 of wavelength λ₁ and output P watts is coupled with one principaloptical fiber 300 via Bragg diffraction optical couplers 302 of whichBragg wavelengths are tuned to λ₁ respectively to create light ofintensity nP watts, which effectively uses the nonreciprocal opticaltransmission property of the Bragg diffraction optical coupler of thepresent invention.

In the optical access system of which representative example is a PDS(Passive Double Star) network system that performs two-way opticalcommunication between a station and N (a plurality) subscribers via 1:Noptical star couplers, the star coupler sufficiently functions in adescending distribution system but ascending signals from the subscriberlines can only collect power of 1/N at the station in the ascendingmultiplexing system, which poses a problem that up signal transmissionloss is great. The optical multiplexer of this embodiment is effectiveas a method of implementing an N:1 optical multiplexer with no loss andcapable of solving such a problem.

Next, an example of configuring an optical circulator is shown as asixth embodiment of the present invention. Here, an example of threeports is described. As for operation of a three-terminal circulator inprinciple, as shown in FIG. 2, it operates so that a signal inputted tothe terminal A is outputted to the terminal B, an input signal to theterminal B is outputted to the terminal C, and an input signal to theterminal C is outputted to the terminal A.

If an optical circulator performing the same operation as this isimplemented by utilizing the nonreciprocity of the Bragg diffractionoptical coupler by the tilt grating of the present invention, it can beconfigured as shown in FIG. 17 as an example 201, 202 and 203 in thedrawing are Bragg diffraction optical couplers supported by two V-groovesubstrates on the topside and underside, where the left terminal of theupper optical fiber 204 of the Bragg diffraction optical coupler 201 isthe input terminal A, the left terminal of the upper optical fiber 205of the Bragg diffraction optical coupler 202 is the input terminal B,and the left terminal of the upper optical fiber 206 of the Braggdiffraction optical coupler 203 is the input terminal C. And it is acircuit connecting the right terminal of the lower optical fiber 207 ofthe Bragg diffraction optical coupler 201 to the right terminal of theupper optical fiber 205 of the Bragg diffraction optical coupler 202,the right terminal of the lower optical fiber 208 of the Braggdiffraction optical coupler 202 to the right terminal of the upperoptical fiber 206 of the Bragg diffraction optical coupler 203, and theright terminal of the upper optical fiber 204 of the Bragg diffractionoptical coupler 201 to the right terminal of the lower optical fiber 209of the Bragg diffraction optical coupler 203.

Next, its operation will be described. Light 210 inputted to theterminal A enters into the Bragg diffraction optical coupler 201, andthen it transmits to the lower optical fiber 207 of the Braggdiffraction optical coupler 201 due to this optical coupler's action. Asthe lower optical fiber 207 of the Bragg diffraction optical coupler 201is connected to the upper optical fiber 205 of the Bragg diffractionoptical coupler 202, this light 210 is inputted to the Bragg diffractionoptical coupler 202 through the optical fiber 205. As the advancingdirection of this light 210 to the Bragg diffraction optical coupler 202is a direction not causing diffraction as aforementioned, the lightadvances as-is to the upper optical fiber 205 to be outputted to theterminal B.

Next, light 211 inputted to the terminal B is outputted to the terminalC on the same route as the one experienced by the aforementioned lightinputted to the terminal A. Likewise, light 212 inputted to the terminalC is outputted to the terminal A, and three-port optical circulatoroperation is implemented as a whole.

As such a Bragg diffraction optical coupler of the present invention hasanother unique characteristic of being able to perform optical wiringamong the devices three-dimensionally, configuration can be easilyperformed by using this characteristic. While the above example shows aninstance of a three-port optical circulator, it is possible to configurean optical circulator of four ports or more terminals and also to createa multiple-port model in advance and change the number of portsafterward by changing optical fiber connections.

Next, a seventh embodiment of the present invention will be described byusing FIG. 18. If the fiber optical amplifier of the second embodimentof the present invention is reviewed from a viewpoint of thenonreciprocity of the Bragg diffraction optical coupler, it isunderstood that a Bragg diffraction optical coupler 72 on the outputside of a fiber optical amplifier 70 not only has a function offiltering signal light 75 from spontaneous emission light 76 of thefiber optical amplifier 70 but also plays a role of the optical isolatorfor preventing reflected light from connection points such as aconnector of output signal light 75, backward scattering light of thewavelengths of signal light itself from the transmission line opticalfiber due to signal light of high intensity and backward scatteringlight of different wavelengths due to optical nonlinear effects of theoptical fiber from returning to the optical amplifier 70 and beingamplified on the input side of the optical amplifier to flow backward.Therefore, if a Bragg diffraction optical coupler 77 is newly providedon the input side of the optical amplifier 70 in addition to it, it ispossible to avoid bad influence on the signal transmission side and therepeater on the front tier exerted by the spontaneous emission light 76also released on the input side of the optical amplifier 70 going backon a signal input fiber to the fiber optical amplifier.

While the description of the tilt grating has referred to a case of asimple grating so far for the purpose of simplification, various methodsof adjusting the transmission wavelength property can be used in theBragg diffraction optical coupler by the tilt grating of the presentinvention for various filters including electricity and not limited tolight. For instance, a chirped grating can be used instead of a singleperiod grating in order to expand a transmission wavelength width. Inaddition, the present invention can also adopt assignment of weights bysuperimposing various window functions such as a Gaussian window inorder to suppress side lobes appearing before and after wavelengths of acentral transmission area. There are two methods thereof, that is, amethod of assigning weights to grating amplitude of the tilt grating inthe direction of optical transmission, and a method of controlling anaccess distance of the two optical fibers comprising the optical couplerin the direction of optical transmission or purposely causing theoptical axes to slightly intersect. Weights can be assigned to gratingamplitude when forming the grating by ultraviolet exposure. In addition,it is possible to assign weights by the access distance of the twooptical fibers, when polishing the plane reaching the core surface, bycontrolling the V groove width supporting the optical fiber shown inFIG. 6 in the direction of the fiber length, for instance, thuscontrolling curvature of bending support of the fiber. Moreover, it ispossible to cause the optical axes to slightly intersect by making anangle on the V groove to be formed with the upper and lower supportingsubstrates of the optical fiber in advance.

Moreover, in the description of the production and assembly method ofthe fiber Bragg diffraction optical coupler by the tilt grating, whileit was mentioned that the V groove substrate supporting the opticalfiber should use a Si wafer, the substrate is not limited to it, and itcan be a glass substrate, a ceramic such as alumina with the V groovemade in advance and sintered, or a metal with the V groove made bycutting work. It is because, as assembly of this device only requiresthat the supporting substrates to be overlaid have the V grooves of thesame arrangement rather than such accuracy of the distances between theV grooves as required by connection between an array fiber and awaveguide array, the above condition is satisfied, even in the case of acut metal substrate of which high accuracy of the row of V groovescannot be expected, by dividing the cut substrate into two for use.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by the present invention is not limited to thosespecific embodiments. On the contrary, it is intended to include allalternatives, modifications, and equivalents as can be included withinthe spirit and scope of the following claims.

1. A method of manufacturing a fiber-type optical coupler, including thesteps of: (a) forming, in an optical fiber having a core surrounded by acladding, a first Bragg diffraction grating by periodic change of arefractive index whereby an angle θ made by a wave vector and an opticalaxis of said optical fiber is 0 degree <θ<90 degrees so as to form afirst Bragg diffraction grating fiber; (b) removing said cladding, atone side, up to a boundary with said first Bragg diffraction grating;and (c) facing the cladding-removed side of said first Bragg diffractiongrating fiber and the cladding-removed side of a second Braggdiffraction grating fiber which has the same configuration as said firstBragg diffraction grating fiber by making the respective optical axesalmost parallel and also making slanting directions of the respectiveBragg diffraction gratings almost parallel and also approximatingsurface planes of the removed claddings of said first and said secondBragg diffraction grating fibers.
 2. The method of manufacturing afiber-type optical coupler according to claim 1, wherein said first andsecond Bragg diffraction grating fibers are accommodated and fixed ingrooves formed on substrates respectively, and said respectivesubstrates have means for placing said first and second Bragg gratingfibers by making the respective optical axes thereof almost parallel andapproximating removed sides of said first and second Bragg diffractiongrating fibers.
 3. The method of manufacturing a fiber-type opticalcoupler according to claim 2, wherein a plurality of said Braggdiffraction grating fibers are fixed on said substrate.
 4. The method ofmanufacturing a fiber-type optical coupler according to claim 3, furthercomprising a step of splicing said plurality of Bragg diffractiongrating fibers.